1. Field of the Invention
Present invention relates to an image processing apparatus and method for converting image data to be outputted to a recording device such as a laser-beam printer, and more particularly, to an image processing apparatus and method for converting the dot density of input image data when the dot density of the input image data is different from the recording dot density of a printer engine.
2. Prior Art
In recent years, a laser-beam printer is widely used as output device of a computer. Especially, a lower-density (e.g., 300 dpi) laser-beam printer is rapidly becoming popular with the merits such as compact size and low cost.
As shown in FIG. 1, a laser-beam printer for 300 dpi (dot per inch) recording density consists of a printer engine 51 and a printer controller 52. The printer controller 52, connected to the printer engine 51, receives a code data from a host computer 54, makes page information consisting of dot data in accordance with the code data and sends the dot data to the printer engine 51 in sequence. The printer engine 51 forms an electrostatic latent image on an electrostatic drum in accordance with the dot data. The latent image is transferred to a recording paper.
The host computer 54 loads an application program from a floppy disk 55 having the application program, The application program is then started, the host computer acts as, e.g., a word processor.
Since various application programs are nowadays produced, an user is able to make a lot of data and preserve them using these application programs.
On the other hand, a printer engine has been improved for the purpose of higher printing quality. Printer engines adapted to a recording density of 600 dpi or more than 600 dpi have been provided. A printer controller connected to such high-density printer engine has a data memory having a capacity corresponding to its recording density (for example, a printer connected to a printer engine for 600 dpi recording density has a memory capacity four times as large as 300 dpi).
Since most of application programs are produced corresponding to printers for lower recording density, they can not be used for printers for higher recording density.
For example, FIGS. 2 and 3 respectively show the dot pattern of letters "G" and "t" for 300 dpi recording density . If these letters are recorded in 600 dpi recording density, the size of these letters become 1/2 in length and in width.
By adopting one of data interpolation methods to the above case, the pattern data for 300 dpi is converted to a dot pattern for 600 dpi, where the length and width of the pattern data for 300 dpi are simply doubled and applied as a pattern data for 600 dpi structure. This method can avoids diminishing the size of letters (FIGS. 4 and 5). However, after the pattern conversion, the jagged edges of the outline portion of the letters remains the same as the jagged edges in FIGS. 2 and 3. This means it is impossible to record image data in high quality corresponding to the ability of 600 dpi printer engine.
Another one of data interpolation methods is known as a smoothing technique where the jagged edges of image data are detected and smoothed. This technique can improve the quality of the image data of characters and figures having at least two continuous dots. However, the technique can not obtain the same effect in a case of recording of picture images processed in Dither method, error diffusion method and the like.
Generally, in an image data processed in a half-tone processing method such as the Dither method and the error diffusion method, in light portions, a black dot whose size is smaller than one dot (300 dpi for 300 dpi printer engine) can not be printed, and a white dot smaller than one dot can not be printed in dark portions. Consequently, the printed image quality of the light portions and the dark portions is lower than that of the neutral density portions.
FIG. 6 shows an example of a fattening type gray scale pattern. FIG. 7 shows an example of a light portion of a half-tone image printed using the gray scale pattern, and FIG. 8, an example of a dark portion of a half-tone image printed using the same. In these examples, isolated dots are expressed as scattered specks, and even if these dots are printed in higher recording density, the size of the smallest speck is the same as the size of a dot in 300 dpi. Therefore, this method can not improve the quality of printed half-tone images.